A Beginner Needs Help!

I am just your everyday photographer, take nature shots, urban life and flowers. I have a Canon 20d. I am familiar with Shutter Speed, Aperture and ISOs and how to control them. Mostly shoot in Aperture Priority mode and let the camera do the rest. A friend of mine asked me to do some portraits of her young daughter. I always wanted to get into portraiture, so I bought two cheap umbrellas, two used flashes (580exii and 430 exii), a long ettl cable along with some stands and a background. I read a lot of the strobist sites, read the two books of Syl and Nick's, and looked at numerous youtube videos. I have figured out how to do a master/slave setup, ratios and such. Still I have some basics that I can't seem to put it together.So every book or blogs that I read, it says "The eyes were too dark, so I repositioned the light" or something like that, or "The background was a little dark during this outside portrait shoot, so I bumped up the shutter speed to bring in more ambient light" and such. Makes sense…BUTAre these famous photographer looking at the LCD screen of the camera to figure out too much darkness around the eyes, or the subject is more lit than the background while they are outside shooting? I am told by many pros including the same writers that you can't really count on the LCD screen for exposure, so do they run to the computer to look at it as they are taking portraits? Or is it experience? It is easy to say I am one stop low in exposure for ambient or flash (so adjust with EC or FEC) when I look at the picture afterwards, but how do you do it during the shoot? Am I missing something (except experience of course!)I plan to put the umbrellas at 45 degrees up, 45 degrees to the sides, set the flashes on ettl with a ratio, set my camera to M (with ISO 100, SP 125 or 250, F 8) and shoot. How do I know I have the eyes that has too much shadow or anything else that would make the portrait bad?Thanks for any advice or help you can provide!Monaz